Center, asymmetric

This band is not seen in normal ultraviolet spectra but can be measured for circular dichroism of 3-R-A-4-thiazoline-2-thione. where R possesses an asymmetric center (74). Representative ultraviolet data are also eiven in Refs. l.S and 75. 

Asymmetric center (Section 7 2) Obsolete name for a chiral ity center... 

Chirality center (Section 7 2) An atom that has four nonequiv alent atoms or groups attached to it At vanous times chiral ity centers have been called asymmetric centers or stereogemc centers... 

When the asymmetric carbon atoms in a chiral compound are part of a ring, the isomerism is more complex than in acyclic compounds. A cyclic compound which has two different asymmetric carbons with different sets of substituent groups attached has a total of 2 = 4 optical isomers an enantiometric pair of cis isomers and an enantiometric pair of trans isomers. However, when the two asymmetric centers have the same set of substituent groups attached, the cis isomer is a meso compound and only the trans isomer is chiral. (See Fig. 1.15.)... 

Isotactic. All substituents lie on the same side of the extended chain. Alternatively, the stereoconfiguration at the asymmetric centers is the same, say, -DDDDDDDDD-. 

Fig. 12. Structures of the mycotrienins, mycotrienols, trienomycins, and ansatrienins where represents configuration of the asymmetric center.

Fig. 1. The four naturally occutting tocopherols (a-tocopherol, E.E.E. [59-02-9] jall-rac [2074-53-5] (1) p-tocopherol [148-03-8] (2) y-tocopherol [54-28-4] (3) 8-tocopherol [119-13-1] (4)), a-tocotrienol [1721 -51 -3] (5), and p-tocotrienol [14101-61-2] (6) where asterisks denote asymmetric centers and the...

Lithium-ammonia reductions of most steroidal enones of interest create one or two new asymmetric centers. Such reductions are found to be highly stereoselective and this stereoselectivity constitutes the great utility of the reaction. For conjugated enones of the normal steroid series, the thermodynamically most stable products are formed predominantly and perhaps exclusively. Thus the following configurations are favored 5a, 8/ , 9a, and in certain cases 14a (see page 35). Starr has listed numerous examples illustrating these facts and Smith " and Barton have tabulated similar data. 

If the ketone function is adjacent to a hydrogen-bearing asymmetric center, the compound can undergo epimerization. In steroids with a normal skeletal configuration (8/3, 9a, 14a) there is no detectable epimerization at C-8 or C-9 during the exchange of and 11- ketones. 

Crude tetrahydropyranyl ethers are usually a mixture of epimers due to formation of an additional asymmetric center. Consequently these derivatives are sometimes difficult to characterize. 

Absolute configurations of the amino acids are referenced to D- and L-glyceraldehyde on the basis of chemical transformations that can convert the molecule of interest to either of these reference isomeric structures. In such reactions, the stereochemical consequences for the asymmetric centers must be understood for each reaction step. Propose a sequence of reactions that would demonstrate that l( —)-serine is stereochemically related to l( —)-glyceraldehyde. 

All of these functions are made possible by the characteristic chemical features of carbohydrates (1) the existence of at least one and often two or more asymmetric centers, (2) the ability to exist either in linear or ring structures, (3) the capacity to form polymeric structures via glyeosidie bonds, and (4) the potential to form multiple hydrogen bonds with water or other molecules in their environment. 

Aldoses with at least three carbons and ketoses with at least four carbons contain chiral centers (Chapter 4). The nomenclature for such molecules must specify the configuration about each asymmetric center, and drawings of these molecules must be based on a system that clearly specifies these configurations. 

D-Methylmalonyl-CoA, the product of this reaction, is converted to the L-isomer by methylmalonyl-CoA epunerase (Figure 24.19). (This enzyme has often and incorrectly been called methylmalonyl-CoA racemase. It is not a racemase because the CoA moiety contains five other asymmetric centers.) The epimerase reaction also appears to involve a carbanion at the a-position (Figure 24.20). The reaction is readily reversible and involves a reversible dissociation of the acidic a-proton. The L-isomer is the substrate for methylmalonyl-CoA mutase. Methylmalonyl-CoA epimerase is an impressive catalyst. The for the proton that must dissociate to initiate this reaction is approximately 21 If binding of a proton to the a-anion is diffusion-limited, with = 10 M sec then the initial proton dissociation must be rate-limiting, and the rate constant must be... 

Tire presence of two different substituents on the ring carbon atom of dithiirane 1-oxides provides two asymmetric centers, i.e., four stereoisomers. 

The [2 + 2] cycloaddition reaction of A -benzyl-l,4-dihydropyridine 34b with acrylonitrile, followed by catalytic reduction gave two pairs of diastereoisomeric amides 36 and 37 with a low diastereomeric excess, probably due to the large distance between the asymmetric center and the site of acrylonitrile attack. Compounds 36 and 37 were resolved into the four individual diastereoisomers (ca 5% for compound 36 and 15% for 37) [97JCR(M)321], Irradiation of 1,4-dibenzyl-1,4,5,6-tetrahydropyridine 38 in the presence of 29 gave two stereoisomers. 

Total synthesis of brefeldins, 13-member lactones with four asymmetric centers 97YGK110. 

The 1,3-dipolar cycloaddition reaction of nitrones with alkenes gives isoxazolidines is a fundamental reaction in organic chemistry and the available literature on this topic of organic chemistry is vast. In this reaction until three contiguous asymmetric centers can be formed in the isoxazolidine 17 as outlined for the reaction between a nitrone and an 1,2-disubstituted alkene. The relative stereochemistry at C-4 and C-5 is always controlled by the geometric relationship of the substituents on the alkene (Scheme 8.6). 

With this reaction, two new asymmetric centers can be generated in one step from an achiral precursor in moderate to good enantiomeric purity by using a chiral catalyst for oxidation. The Sharpless dihydroxylation has been developed from the earlier y -dihydroxylation of alkenes with osmium tetroxide, which usually led to a racemic mixture. 

It is quickly deacylated in vivo and may qualify as a prodrug. The published synthesis is rather long and bears conceptual similarities to the synthesis of cannabinoids. It has some five asymmetric centers. Dane salt formation between 3,5-di-methoxyani1ine and ethyl acetoacetate followed by borohydride reduction gives synthon The amino group is protected by... 

Molecules that contain two or more asymmetric centers exist in more than two stereoisomeric forms. Some are pairs of optically active isomers others may be symmetric and therefore optically inactive. An example is tartaric acid with two asymmetric central carbon atoms. It has three isomers, two of which are optically active and one inactive. 

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